While the idea of a "cis" to "trans" conformation
change in proline (or vice versa) as a molecular switch is a nice idea,
as far as I know it has never been been rigorously tested for the nicotinic
receptor. Interestingly, a cis-trans iosomerization at a proine has been shown
to open the pore of a ligand-gated ion channel closely related to the nicotinic
receptor (Nature 438: 248-252, 2005).